The present invention relates to butadiene/isoprene/monovinyl aromatic monomer heptablock copolymers and methods for the preparation thereof, More particularly, the present invention relates to block copolymers having the following symmetric structures: Z-XZ-X-Y-X-XZ-Z, wherein Z represents a polymer block of monovinyl aromatic monomer, X and Y, being different from each other, represent polymer blocks of butadiene or isoprene and XZ represents a tapered copolymer block of monovinyl aromatic monomer and butadiene or isoprene.
Generally, representative block copolymers based on butadiene, isoprene and styrene are SBS and SIS, with SBS being butadiene/styrene triblock copolymers (wherein B represents polybutadiene block and S represents polystyrene block), and SIS being isoprene/styrene triblock copolymers(wherein I represents polyisoprene isoprene block and S represents polystyrene block). By employing difunctional lithium based initiators and changing the charging orders of butadiene, isoprene and styrene, block copolymers having various different structures can be obtained. Such block copolymers comprise those having the following symmetric structures: (1) S-I-BI-B-BI-I-S(with butadiene and isoprene being charged all at once and then styrene being separately charged); (2)S-I-B-I-S(with butadiene, isoprene and styrene being charged in that order), as disclosed in EP-0 413 294A2; (3) S-B-I-B-S(with isoprene, butadiene and styrene being charged in that order).
The object of the present invention is to prepare multiblock copolymers based on butadiene, isoprene and monovinyl aromatic monomers, such multiblock copolymers possessing the excellent properties of both SBS and SIS, for example, excellent tensile strength, elongation at break and the like, and being thermoplastic elastomeric materials having various utilities. After being hydrogenated, such multiblock copolymers can be used for preparing heat-sensitive adhesives and pressure-sensitive adhesives, and provide advantages to which general SBS""s and SIS""s are incomparable.
The present invention, it one aspect, provides a family of butadiene/ isoprene/monovinyl aromatic monomer heptablock copolymers, with the single monomer usually used for forming B blocks in conventional SBS""s or I blocks in conventional SIS""s being replaced by two monomers, i.e. butadiene and isoprene, and moreover, I blocks or B blocks existing together with BS or IS tapered copolymer blocks.
The present invention, in another aspect, provides a method for preparing the above heptablock copolymers, characterizing by firstly substantially completely polymerizing one monomer of butadiene and isoprene by using difunctional lithium based initiators, then charging a mixture consisting of the other monomer of butadiene and isoprene and monovinyl aromatic monomers into the resulting living polymer and substantially completely polymerizing them.
The butadiene/isoprene/monovinyl aromatic monomer heptablock copolymers in accordance with the present invention have the following symmetric structures: Z-XZ-X-Y-X-XZ-Z, wherein Z represents a polymer block of monovinyl aromatic monomer, X and Y, being different from each other, represent polymer blocks of butadiene or isoprene and XZ represents a tapered copolymer block of monovinyl aromatic monomer and butadiene or isoprene, and wherein the content of the recurring units derived from monovinyl aromatic monomers is 10 to 50 percent by weight, preferably 20 to 40 percent by weight, based on the weight of the block copolymer, the content of the recurring units derived from butadiene is 10-75 percent by weight, preferably 20 to 50 percent by weight, based on the weight of the block copolymer and the content of the recurring units derived from isoprene is 10-75 percent by weight, preferably 20 to 50 percent by weight, based on the weight of the block copolymer.
Preferably, the butadiene/isoprene/monovinyl aromatic monomer heptablock copolymers in accordance with the present invention have a number-average molecular weight(Mn, determined by Gel Permeation Chromatographic(GPC) method) of 5xc3x97104 to 50xc3x97104, more preferably 10xc3x97104 to 30xc3x97104, and most preferably 10xc3x97104 to 25xc3x97104.
In the butadiene/isoprene/monovinyl aromatic monomer heptablock copolymers in accordance with the present invention, the content of 1,2-polybutadiene structure is typically from 6 to 35 percent by weight, preferably from 10 to 20 percent by weight, based on the weight of polybutadiene block and the content of 3,4-polyisoprene structure is typically from 6 to 35 percent by weight, preferably from 10 to 20 percent by weight, based on the weight of polyisoprene block. There is no particular restriction with respect to the manners for controlling the contents of 1,2-polybutadiene structure and 3,4-polyisoprene structure within the above ranges, respectively, and they can be achieved simply by polymerizing said monomers in non-polar hydrocarbon solvents by conventional anionic polymerization processes. In this regard, for example, J. Fetters et. al. disclosed, in Adv. In Polymer Sci., 56, 28(1984), that when employing alkyl lithium as initiator to polymerize isoprene in non-polar solvents, the resulting polyisoprene has the following microstructures: 3,4-addition structures are about 5 to 8 percent, and the remainders are predominantly cis-1,4-addition structures, together with small amounts of trans-1,4-addition structures, depending on the type of solvent and the polymerization temperature used. Polybutadiene usually has three structures, i.e. cis-1,4-, trans-1,4- and 1,2-addition structures, and when butadiene is polymerized in non-polar solvents by employing alkyl lithium as initiator, the resulting polybutadiene has a low content of 1,2-addition structures, typically about 10%.
The monovinyl aromatic monomers which are preferred in the present invention includes styrene or alkyl substituted styrene such as vinyl toluene(all isomers, alone or in admixture), xcex1-methylstyrene, 4-tertiary-butylstyrene, 4-methylstyrene, 3,5-diethylstyrene, 3,5-di-n-butylstyrene, 4-(4-phenyl-n-butyl) styrene, 2-ethyl4-benzylstyrene, 4-cyclohexylstyrene, 4-n-propylstyrene, 4-dodecylstyrene and mixtures thereof. More preferable monovinyl aromatic monomers are styrene, vinyltoluene, xcex1-methylstyrene and mixtures thereof, with styrene being most preferred.
In the method for preparing the heptablock copolymers in accordance with the present invention, the polymerization of the foregoing monomers is conducted according to well known anionic polymerization conditions. Suitably, the monomers are polymerized in a solvent at a temperature of 20xc2x0 C. to 90xc2x0 C., preferably 30xc2x0 C. to 80xc2x0 C. Suitable solvents include non-polar hydrocarbon solvents, selected from the group consisting of non-polar aromatic hydrocarbons, non-polar aliphatic hydrocarbons and mixtures thereof. Examples of solvents include benzene, toluene, ethylbenzene, xylene, pentane, hexane, heptane, octane, cyclohexane, mixed xylene, raffinate oil or mixtures thereof. In the method according to the present invention, the concentration of the monomer to be firstly polymerized, either butadiene or isoprene, in the solvent used is preferably from 10 to 20 percent by weight.
In the method for preparing the heptablock copolymers in accordance with the present invention, the amount of the monovinyl aromatic monomer used is from 10 to 50 percent by weight, preferably from 20 to 40 percent by weight, based on the total weight of the monomers; and the amounts of butadiene and isoprene used are from 10 to 75 percent by weight, preferably from 20 to 50 percent by weight, respectively, based on the total weight of the monomers.
The initiator useful in the present invention is difunctional lithium based initiators, selected from the group consisting of bislithiums derived from dihalogenated alkanes and oligomeric bislithiums thereof, having the general formulae LiRLi and Li(DO)nR(DO)nLi, respectively, wherein R represents alkyl groups having from 4 to 10 carbon atoms, DO represents a conjugated diene having from 4 to 8 carbon atoms or mixtures thereof, with 1,3-butadiene and isoprene being preferred and n represents the oligomerization degree, typically ranging from 2 to 8, preferably from 3 to 6; bislithiums of naphthalene; and bislithiums derived from diene compounds and oligomeric bislithiums thereof. Examples of bislithiums derived from dihalogenated alkanes and oligomeric bislithiums thereof include 1,4-dilithiobutane, 1,2-dilithio-1,2-diphenylethane, 1, 4-dithio-1,1,4,4tetraphenylbutane, 1,4-dilithio-1,4-dimethyl-1,4-diphenyl-butane and isoprene oligomer-bislithiums and butadiene oligomer-bislithiums thereof; examples of bislithiums of naphthalene include dilithionaphthalene and xcex1-methyl-dilithio-naphthalene; and examples of bislithiums derived from diene compounds and oligomeric bislithiums thereof include 1,3-phenylene-bis[3-methyl-1-(4-methyl)-phenylpentylidene ]bislithium, 1,3-phenylene-bis[3-methyl-1-(4-methyl)phenylpentylidene ]butadiene oligomer-bislithium, 1,3-phenylene-bis[3-methyl-1-(4-methyl) phenylpentylidene]isoprene oligomer-bislithium, 1,4-phenylene-bis[3-methyl-1-(4-methyl) phenylpentylidene]bis-lithium, 1,4-phenylene-bis[3-methyl-1-(4-methyl) phenylpentylidene]butadiene oligomer-bislithium, and 1,4-phenylene-bis[3-methyl-1-(4-methyl) phenyl-pentylidene]isoprene oligomer-bislithium. The above initiators can be used alone or in admixture.
The amount of the initiator used depends on the desired number-average molecular weight of the block copolymers. In the present invention, the difunctional lithium based initiator is used in such an amount that the block copolymers have a number-average molecular weight of 5xc3x97104 to 50xc3x97104.
After conducting anionic polymerization as described above, it is preferred that to the resulting polymer liquid is added an antioxidant or a mixture thereof, for example a mixture of Irganox 1010(trade name, available from Ciba-Geigy AG, Switzerland) and ANTIGENE BHT(trade name, 2,6-di-tertiary-butyl-4-methylphenol, available from Sumitomo Chemical Co., Ltd., Japan) in a weight ratio of 1:1, and then the polymer liquid is post-treated by conventional manners and subjected to analysis after being dried.